Si:N DISTROPHY AND ITS CONSEQUENCES FOR COASTAL PHYTOPLANKTON: MODULATION BY P AVAILABILITY

The north-western Black Sea is a coastal area where eutrophication has caused severe damages to the functioning of the pelagic and benthic ecosystems. It has been suggested that increasing delivery of inorganic N and P along with decreasing inputs of Si have led to an increase in the frequency and magnitude of phytoplankton blooms, nanoplanktonic species progressively replacing diatoms.
Within the framework of the EU EROS project, phytoplankton dynamics has been studied by means of 14C, 15N, 32Si and 32P experiments in the Danube - Black Sea mixing area. These process studies have shown that the situation is much more complicated than the one described above. Near the Danube river mouth, low salinity processes such as important nutrient uptake by diatoms and nitrification induce high values of the N :P ratio in the coastal area, leading to P limitation of diatom growth and to the dominance of dinoflagellates early in the productive season. Later, this system switches to the dominance of diatoms under a N-limited regenerated regime. Further south, P limitation is relieved owing to important inputs from the Navodari industry and the situation is likely to conform to the Si hypothesis of Smayda (1990). In both areas, P availability may exert a primary control on the effects of Si :N distrophy in terms of phytoplankton composition and food web structure.